Process for selective separations of plutonium from uranium and other metals

ABSTRACT

A process for selectively separating plutonium from uranium and other metals is disclosed wherein, in order to precipitate and remove the pertaining complexes, use is made of a sufficient quantity of a cationic compound containing at least one group adapted to assert affinity to polar surfaces and containing a radical which has little affinity to water, e.g. surface active agents and the like, and use is made of the capability of Pu 4+  and U 4+  to form nitrato-complexes and of the capability of Pu 4+  and UO 2   2+  to form sulfato-complexes.

This application is a division of application Ser. No. 680,188 filedDec. 10, 1984 which, in turn, is a division of application Ser. No.509,193 filed June 29, 1983.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to improvements in or relatingto processes for the selective separation of plutonium and uranium fromone another and from other metals.

Nowadays, the processes of jointly separating plutonium and uranium fromother metals and the separation of plutonium from uranium are almostalways carried out using aqueous solutions. There are presentlyprimarily two methods of particular interest, with each being used inseveral variations: the solvent extraction method and the ion exchangemethod. While both methods can, in general, be employed--also on acommercial scale--both still exhibit undesireable disadvantages.

Relative long residence times of organic solvents, agents adapted toform complexes, or resins, in higher radiation density ranges, lead toradiolytical destruction and, thereby, to losses in capacity anddetrimental gas formation.

In the case of solvent extraction methods, the penetration of UO₂ ²⁺ andPu⁴⁺ through the phase boundary between water and organic solvent, aswell as the concentration of precipitates at the phase boundary orinterface, present kinetic problems.

When using the mixer-settler technique during solvent extraction, thestationary processing is rendered more difficult by density fluctuationsin the organic and aqueous phases during successive extraction andre-extraction steps.

The reaction of ion exchange resins and complex-formers, e.g.tributylphosphate, with concentrated HNO₃ presents an unnecessary dangerpotential.

The separation accuracy of solvent extraction processes, as well as ionexchange processes during the separation of uranium from plutonium, andin the separation of the two elements from one another, is of such a lowextent that the separation is generally carried out in severalsuccessive steps.

These difficulties are already apparent in the case of relatively welldefined solutions obtained, for example, by the dissolution of nuclearfuel elements in concentrated HNO₃. Additional difficulties areencountered in such processing when nonuniform solutions are treated,which possibly also contain difficult to define organic materials offluctuating composition. For such waste waters there have not beenadvanced fully satisfactory treatment processes so that also therecovery of nuclear fuel elements from such waste waters, the transport,and the storage of remaining residues have not been solvedsatisfactorily.

2. Description of the Prior Art

A flotation process for inorganic ions has been described in DE-AS No. 1166 113 (DE-AS=German patent publication) in which process in conformitywith the loading of the ion to be floated, there is added to theion-containing solution an anionic or cationic collector and in whichthe so-formed insoluble reaction product is floated or slurried underintroduction of a gas and removed as a foam or froth. In accordance withthis prior art process the uranium may be separated from uranylsulfatesolutions, albeit with poor yields.

In German Pat. No. 2 817 029 there is disclosed a process forselectively separating and recovering uranium from accompanying metals.In accordance with this prior art process hydrochloric acid is added toan aqueous solution containing uranium until the UO₂ ²⁺ forms anionicchloro-complexes, a surface active agent is added, and the resultantprecipitate is floated in a flotation cell. Uranium is recovered fromthe floated precipitate. While this prior art process allows recovery ofuranium at a higher yield in comparison to the process of German patentpublication No. 1 166 113, its particular drawbacks reside therein thatit is based on the use of hydrochloric acid and, accordingly, can not beutilized on a larger scale without precautionary measures due tocorrosion.

In accordance with German Pat. No. 2 902 516, the process of German Pat.No. 2 817 029 is extended to solutions containing sulfuric acid. Thisprovides yields and concentrations of uranium which are superior incomparison with the process of German patent publication No. 1 166 113.However, the flotation time is longer and the concentration or enrichingis lower than suggested by the process according to German Pat. No. 2817 029; on the other hand, the corrosion dangers have been reduced.

In any event, the foregoing references do not provide a teaching of howto separate plutonium from uranium by means of particular organiccompound-assisted precipitation.

SUMMARY OF THE INVENTION

There has continued to remain, therefore, a need to provide a new orimproved process of separating uranium and plutonium which process doesnot exhibit the drawbacks indicated briefly in the foregoing, or only toa lesser extent.

In general terms, it was investigated whether plutonium could beprecipitated from aqueous solutions as a substantially insolubleprecipitate, either as a cation or as complex anion, using eitheranionic or cationic compounds containing at least one group adapted toassert affinity to polar surfaces and also containing a radical whichhas little affinity to water, whereby the plutonium could be separated,in this manner, selectively from uranium or other elements. It wasfurther investigated whether plutonium precipitated in this manner wouldlead to hydrophobic precipitates which could be rapidly removed from themother liquor by means of ionic flotation, or precipitation-flotation,respectively.

It is an object of the present invention to provide a process forseparating plutonium and uranium which is simple, provides a method forthe recovery of plutonium from nuclear-plant wastes and otherradioactive wastes, and is devoid of the disadvantages of the prior artprocesses.

In accordance with one aspect of the present invention there is provideda process for selectively separating plutonium and uranium comprisingthe steps of: adding nitric acid in certain amounts to a solutioncontaining Pu⁴⁺ and UO₂ ²⁺, with the amount of nitric acid added beingsufficient such that the Pu⁴⁺ forms anionic nitrato-complexes, adding,prior to, or with, or during, addition of the nitric acid, a compoundcontaining at least one group adapted to assert affinity to polarsurfaces and also containing a radical which has little affinity towater, separating the resultant precipitate from its mother liquor, andrecovering the plutonium from the precipitate.

In accordance with another aspect of the present invention there isprovided a process for selectively separating plutonium and uraniumwhich comprises the steps of: adding, to a solution containing Pu⁴⁺ andUO₂ ²⁺, sulfuric acid in such an amount that the uranium forms anionicsulfato-complexes, adding a sufficient amount of a cationic compoundcontaining at least one group adapted to assert affinity to polarsurfaces and also containing a radical which has little affinity towater, separating the resultant precipitate from its mother liquor,adjusting, after separation of the uranium-containing precipitate, thepH value in the remaining mother liquor in such a way that the Pu⁴⁺forms anionic sulfato-complexes, adding a further amount of saidcationic compound, separating the resultant precipitate from its motherliquor, and recovering plutonium from the precipitate.

In accordance with a further aspect of the invention there is provided aprocess for selectively separating plutonium and uranium which comprisesthe steps of: adding to a solution containing Pu⁴⁺ and UO₂ ²⁺ such anamount of sulfuric acid that Pu⁴⁺ as well as UO₂ ²⁺ form anionicsulfa-to-complexes, next adding iron(II)sulfate or other suitablereducing agents in such an amount the Pu⁴⁺ present is reduced to Pu³⁺,adding a cationic compound containing at least one group adapted toassert affinity to polar surfaces and also containing a radical whichhas little affinity to water, separating the resultant precipitate fromits mother liquor, and recovering uranium from the precipitate. Afterseparation of the uranium-containing precipitate, the process iscontinued by adding to the remaining mother liquor either sodium nitriteor peroxodisulfate, or other oxidizing agents, in such an amount thatthe Pu³⁺ is re-oxidized, adding a further amount of said cationiccompound, separating the resulting plutonium-containing precipitate, andrecovering the plutonium from the precipitate.

Electrolytic methods can be employed correspondingly instead of thereducing agents during the reduction and/or the oxidizing agents duringthe re-oxidation, respectively.

In accordance with yet another aspect of the present invention there isprovided a process for selectively separating plutonium and uraniumwhich process comprises the steps of: adjusting the sulfuric acidconcentration and the pH of a solution containing Pu⁴⁺ and UO₂ ²⁺ insuch a way that both elements form anionic sulfato-complexes, adding acationic compound containing at least one group adapted to assertaffinity to polar surfaces and also containing a radical which haslittle affinity to water, separating the resultant precipitate from itsmother liquor, suspending the precipitate in nitric acid forre-dissolution of the precipitated uranium, and isolating plutonium fromthe remaining precipitate and uranium from the uranium-containingsolution.

In accordance with the present invention the exposure time of theutilized organic material is to be held at a minimum when exposed inranges of high radiation densities or fluxes. Furthermore, the amount ofthe compound used is to be minimized. A process in accordance with thepresent invention is to be carried out exclusively in an aqueousenvironment so as to avoid problems of phase penetration kinetics (phasetransfer kinetics) and density fluctuations of a two-phase system.

In continuation of the efforts of isolating, using flotation andenriching or concentration techniques, UO₂ ²⁺ from homogeneous aqueoussolutions by means of cationic compounds containing at least one groupadapted to assert affinity to polar surfaces and also containing aradical having little affinity to water, as suggested in German Pat. No.2 817 029, the possibility was examined to apply such technique in theisolation and enriching or concentration of Pu⁴⁺. In this context, therewas utilized the known finding that Pu⁴⁺ and U⁴⁺ are capable of formingsulfato-complexes, as well as nitrato-complexes and, accordingly,satisfy the essential requirement for precipitation-flotation using suchcationic compounds.

It was surprisingly found that Pu⁴⁺ and U⁴⁺ can be precipitated asnitrato-complexes using the cationic compounds referred to, with apreferred compound comprising cetylpyridinium, from nitric acid ornitrate-containing solutions at concentrations of NO₃ ⁻ of less than 10M, whereas UO₂ ²⁺ would not be precipitated. This is even moresurprising since one had to conclude, on the basis of the bondingcapability of anionic exchange resins for uranium in nitric acidsolutions, that UO₂ ²⁺ would also be capable to form nitrato-complexes.

It was furthermore surprising that the cationic compounds used forprecipitation, particularly the alkyl-pyridinium salts, exhibit a highstability under the influence of radioactive radiation. Thus, radiationdoses of up to 10⁶ rad did not cause a detectable influence on theprecipitation characteristics or capability of the cetylpyridiniumcation. Only at 10⁸ rad could be noted a significant, approximately 30%,decrease of the precipitation capability.

Thus, in accordance with the invention there is provided a simple andhighly selective process for the separation of Pu⁴⁺ and UO₂ ²⁺ and forthe recovery of both ions from aqueous solutions. In accordance with thepresent invention it is not required that organic solvents or ionexchange resins are used. Instead highly radiation-resistant reagentsare used, and the residence time of these in the solution is,furthermore, relatively short. In accordance with the invention, one canseparate Pu⁴⁺ from UO₂ ²⁺ and associated metals, in nitric acidsolutions, provided the associated metals do not form nitrato-complexes,or only form anionic nitrato-complexes less stable than Pu⁴⁺, and whichdo not precipitate an addition of said cationic compounds.

Pu⁴⁺ as well as UO₂ ²⁺ form, in sulfuric acid solutions and insulfate-containing solution, sustantially insoluble precipitates onaddition of the cationic compounds mentioned before. Thus, Pu⁴⁺ and UO₂²⁺ can either jointly or individually be separated from all associatedmetals in such solutions which do not form sulfato-complexes, or onlysulfato-complexes which are less stable than Pu⁴⁺ and UO₂ ²⁺ and whichdo not precipitate on addition of cationic compounds of the classdescribed.

Thus, in accordance with the present invention there are indicatedseveral methods for the separation of uranium and plutonium: Pu⁴⁺ isprecipitated as a substantially insoluble complex from a solutioncontaining Pu⁴⁺ and UO₂ ²⁺ in the presence of nitric acid. The UO₂ ²⁺remains in solution and it can be precipitated either by the addition ofsulfate ions as sulfato-complex, or upon reduction to U⁴⁺ asnitrato-complex.

Pu⁴⁺ and U⁴⁺ are together precipitated from nitric acid solution. Theprecipitate is subjected to a selective oxidation such that U⁴⁺ isconverted to UO₂ ²⁺ and is re-dissolved in the solution.

UO₂ ²⁺ and Pu⁴⁺ are precipitated together from sulfate-containingsolutions, and subsequently UO₂ ²⁺ is re-dissolved from the precipitateby nitric acid.

In a sulfate-containing solution of both metals the Pu⁴⁺ is reduced toPu³⁺, and the UO₂ ²⁺ is precipitated as substantially insoluble complexby cationic compounds of the class indicated above. After re-oxidationof the Pu³⁺ to Pu⁴⁺, Pu⁴⁺ is then also precipitated.

In a sulfate-containing solution also containing Pu⁴⁺ and UO₂ ²⁺ the pHvalue is adjusted in two steps in such a way that initially one elementforms a substantially insoluble precipitate, and then the other, onaddition of cationic compounds of the class indicated herein, followedby separation of the materials of interest.

In nitric acid solution, Pu⁴⁺ is reduced to Pu³⁺ and UO₂ ²⁺ is reducedto U⁴⁺. The U⁴⁺ is then precipitated by use of a cationic compound. ThePu³⁺ is re-oxidized to Pu⁴⁺ and is also precipitated.

The lower limit of the concentration of nitric acid is from between 0.1to 1.0 N HNO₃. An upper limit could not be determined to-date. It wasfound that one could use concentrations of 10 N HNO₃ .

At concentrations of HNO₃ of less than 10 N, UO₂ ²⁺ forms a complex UO₂NO₃ ⁺. At a concentration of HNO₃ of greater than 10 N, there resultanionic complexes (through adsorption of uranium on anionic exchangers),but the composition of these complexes is unknown. In HNO₃, Pu⁴⁺ formsall complexes from Pu(NO₃)³⁺ to Pu(NO₃)₆ ²⁻ compare: I. M. Cleveland,The Chemistry of Plutonium, American Nuclear Society). Pu³⁺ does notform anionic nitrato-complexes.

The cationic compound referred to herein is optimally added at slightlyhigher concentrations than stoichiometric concentrations, approximatelyat an excess of 1.01 to 1.05. When one adds a substantially greateramount of such cationic compound, of course, more metal enters theprecipitate, however, unnecessary intensive foaming or frothing occurs,and the duration of the flotation process is extended. Furthermore, theflotation characteristics of the precipitate deteriorate.

If flotation is not desired, but instead centrifuging or filtration isused, the cationic compound may be used at higher concentrations, e.g.1.01 to 1.50 equivalence.

U an Pu can be separated from the majority of radioactive fissionproducts which are generated in a reactor. The separation is easy whenthe other metals do not form anionic complexes and/or anionicpoly-acids, or heteropoly-acids, respectively. The separation of anioniccomplexes and/or poly-acids is achieved in substantially all cases aswell when one either somewhat varies ligand concentration (NO₃ ⁻, SO₄²⁻), or the pH, or U or Pu, respectively, or selectively oxidizes orreduces, respectively, the other metals. Thus, even the separation of Pufrom Ce (which represents a simulate for Pu and also represents animportant fission product) by selective reduction of the Ce⁴⁺ to Ce³⁺can be achieved. The Ce³⁺ does not form precipitates on addition ofcationic compounds as referred to.

Of primary importance is the separation of Pu and U from nitric acid ornitrate-containing solutions. On precipitation with appropriate cationiccompounds as outlined herein, aside from Pu⁴⁺, only the ions Ce⁴⁺ Ru²⁺,Mo⁶⁺, U⁴⁺ and Th⁴⁺ precipitate. Depending on the given problem, theseelements can either be separated by selectivity in the precipitationconditions, or by a subsequent process, from the plutonium, or they mayalso be left with the plutonium.

The invention is generally applicable to the separation of U and Pu fromany starting materials which contain U and Pu. Thus, both elements wereremoved from two types on non-specific, medium-active-waste waters(obtained from the Nuclear Research Center Karlsruhe, Germany):

1. From solutions which are collected in a central collecting tank andemanate from various laboratories. On average, the composition of suchwaste waters is somewhat constant. With good approximation this solutioncontains: Na⁺, Al³⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Mg²⁺, Mn²⁺ MoO₄ ²⁻, Ni²⁺,Zn²⁺, and ZrO²⁺ in 1 M NHO₃, from which plutonium and, as required, alsouranium are recovered.

2. From solutions which come from wet-incinerated Pu-containing wastes(rubber gloves, crucible tongs, towels, filter paper and the like) ofthe production of plutonium oxide. These are sulfuric acid solutions (1to 2 M H₂ SO₄) which, aside from plutonium and, possibly, uranium, alsocontain Al³⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sn⁴⁺, TiO²⁺, andZn²⁺.

With respect to other operating conditions and techniques reference maybe had to German Pat. No. 2 817 029.

The following examples serve to further illustrate the invention.

EXAMPLE 1 Selective Precipitation of Pu⁴⁺ as Nitrato-Complex in thePresence of UO₂ ²⁺

A solution was produced containing 625 mg UO₂ ²⁺ and 25 mg Pu⁴⁺ in 100ml of 7.0 M HNO₃. Solid cetylpyridinium cholride, 4 g, was dissolved inthe solution. After several minutes a green precipitate of fine crystalswas obtained. This was stirred for 30 minutes and subsequently filtered.

EXAMPLE 2 Joint Precipitation of Pu⁴⁺ and UO₂ ²⁺ as Sulfato-Complexesand Seperation in HNO₃

In 75 mg of 1.3 M H₂ SO₄ were dissolved 25 mg Pu⁴⁺, containing also Pu⁶⁺and Pu³⁺, and 635 mg UO₂ ²⁺, and 1 g solid NaNo₂ was added. Upondissolution of the salt, the pH of the solution was adjusted to 2.5 bythe addition of aqueous ammonia, and water was added to bring thesolution to 100 ml. In this solution 4 g solid cetylpyridinium cholridewere dissolved. After several minutes a yellow precipitate of finecrystals was obtained which was filtered after stirring for 30 minutes.

The filtrate contained only 1.13 % of the initially dissolved Pu⁴⁺ and0.8 % of the initially dissolved UO₂ ²⁺. The recovery of plutonium inthe precipitate was, accordingly, 98.78 % and that of uranium 99.19 %.

The precipitate was then suspended in 10 ml of 1.0 M HNO₃. Theuranium-containing part of the precipitate was dissolved, while theplutonium-containing part remained in suspension. The precipitate wasseperated by centrifuging, and the uranium was removed from the motherliquor by neutralizing with aqueous ammonia, in the form of ammoniumdiuranate.

EXAMPLE 3 Selective Precipitation of UO₂ ²⁺ as Sulfato-Complex in thePresence of Pu⁴⁺

In 75 ml of a 1.3 M H₂ SO₄ solution were dissolved 24 mg Pu⁴⁺ and 632 mgUO₂ ²⁺, and 152 mg solid FeSO₄ was added. Upon dissolution of the saltthe pH was adjusted to 2.5 by the addition of aqueous ammonia, and waterwas added to bring the solution to 100 ml. The solution was subsequentlytreated as described in EXAMPLE 2. The uranium reported at 99.46 % inthe precipitate, and the plutonium reported at 97.8 % in the filtrate.

EXAMPLE 4 Selective Precipitation of Pu⁴⁺ as Nitrato-Complex in thePresence of UO₂ ²⁺ and Am³⁺

A solution was produced containing 37.2 μg Pu⁴⁺, 3.66 mg UO₂ ²⁺, and anunknown but relatively small amount of Am³⁺, in 100 ml of 7.0 M HNO₃. Tothe solution was added 120 mg solid cetylpyridinium chloride whichdissolved rapidly. The subsequent, green precipitate was stirred for 30minutes and then filtered. The precipitate contained 91 % plutonium, anduranium and americium reported at 97.7 % each in the filtrate.

EXAMPLE 5 Fractionated Precipitation of UO₂ ²⁺ and Pu⁴⁺ by Varying thepH Value

In 200 ml 1.0 M H₂ SO₄ were dissolved 25 mg Pu⁴⁺ and 635 mg UO₂ ²⁺, andthe pH value was adjusted to 1.5 by the addition of aqueous ammonia.After addition of 3.8 g cetylpyridinium chloride and stirring for 30minutes, the resultant precipitate was filtered and analyzed. Itcontained 98.2 % of the initial uranium content and less than 1 % of theentire Pu-activity. The pH of the filtrate was adjusted to 3.0 by theaddition of aqueous ammonia. The plutonium was precipitated by a furtheraddition of 100 mg cetylpyridinium chloride. The plutonium-containingprecipitate was filtered and analyzed. It contained 97.9 % of theinitial Pu-activity and only traces of uranium.

EXAMPLE 6 Fractionated Precipitation of Plutonium and Uranium fromNitric Acid Solutions

A solution of 625 mg UO₂ ²⁺ and 25 mg Pu⁴⁺ in 100 ml 4.3 M HNO₃ wasprepared. To this solution was added a solution of 0.1 g cetylpyridiniumchloride in 3 ml of 4.3 N HNO₃. After a few minutes a green precipitateof fine crystals was formed which was stirred for 30 minutes and thenfiltered. The filtrate was added to a sufficient amount of hydrazine,sufficient to produce a 0.2 molar hydrazine solution, and it was thenintroduced into a special electrolysis cell. The uranium waselectrolytically reduced from UO₂ ²⁺ to U⁴⁺. The reduction was carriedout using a platinum screen as anode and mercury as cathode, at avoltage of 4.5 V. The 4-valent uranium was subsequently precipitated byaddition of a further 4 g of cetylpyridinium chloride (dissolved in 10ml 0.3 N HNO₃ at 60° C.), and the suspension was again stirred for 30minutes. Upon filtration of the uranium-containing precipitate, thefiltrate was evaporated, and the residue as well as the twometal-containing precipitates, were analyzed. The plutonium precipitatecontained 97.8 % of the starting plutonium-activity and only traceamounts of uranium. The uranium precipitate contained 94.3 % of thestarting uranium content and less than 1 % of the startingplutonium-activity.

In this invention, the terminology: a compound containing at least onegroup adapted to assert affinity to polar surfaces and also containing aradical having little affinity to water, is intended to convey themeaning of the German chemical expression TENSID. The expression isderived from the Latin and suggests `tensioned`. According to the 6thEdition of the German chemical encyclopedia `Rompp's Chemie Lexicon`,the definition given therein embraces at least portions of and all ofthe following terms and agents: surface active agents,phase[-surface]-active agents, detergents, surfactants, syndets and thelike. This definition of TENSID has been proposed at the session of theCommission Internationale de Terminologie des Comite International de laDetergence, on Feb. 24, 1960, in Luzern, Switzerland.

Reference in this disclosure to details of the specific emobidments isnot intended to restrict the scope of the appended claims, whichthemselves recite those features regarded as essential to the invention.

We claim:
 1. A process for the selective separation of plutonium anduranium which comprises: adding sulfuric acid to a solution containingPu⁴⁺ and UO₂ ²⁺ and adjusting to pH thereof to a value at which a UO₂ ²⁺-sulfate complex is formed in the substantial absence of concomitantformation of a Pu⁴⁺ -sulfate complex; adding to said solution a cationiccompound containing at least one group adapted to assert affinity topolar surfaces and also containing a radical which has little affinityto water whereby a first precipitate is formed from said UO₂ ²⁺ -sulfatecomplex; separating said first precipitate; adjusting the pH of theresidual solution to a value at which at Pu⁴⁺ -sulfate complex isformed; forming a second precipitate from said Pu⁴⁺ -sulfate complexwith a cationic compound having characteristics as above defined;separating said second precipitate; and recovering plutonium from saidsecond precipitate.
 2. A process according to claim 1 in which saidamount of cationic compound added to said solution is sufficient to formboth said first precipitate and said second precipitate.
 3. A processaccording to claim 1 in which said amount of cationic compound added tosaid solution is at least sufficient to form said first precipitate, andan additional amount of said cationic compound is added to said residualsolution after the formation of said Pu⁴⁺ -sulfate complex to form saidsecond precipitate.